Abstract

The statistical aspects of laser‐induced breakdown of transparent dielectrics are reexamined and it is found that the experimentally observed independence of the damage probability on spot size is inconsistent with previous theory. In an attempt to find spot‐size‐independent contributions to damage statistics, modification of the electronic properties of the material caused by intense photon fluxes below the one‐shot damage threshold is considered. This modification takes the form of absorbing defects (color centers). The case of high‐quality NaCl (containing 5×1016 Cl−vacancies) is discussed in detail. At the ruby frequency, it may require up to three identical 60‐nsec shots at the same sample site to cause damage at field strengths below the one‐shot threshold. The results exhibit the proper volume dependence and they indicate that laser‐induced material modifications prior to breakdown may indeed contribute to damage statistics.